EPHANIAH preached in Judah during the days of the last good king to reign in Jerusalem. The king's name was Josiah. Josiah (Zephaniah's cousin, because Hezekiah was their common ancestor) led a revival in Judah. And in fact, Josiah's righteous acts were forecasted by an unnamed prophet over 3 centuries prior to his reign (1st Kings 13:2). So why is Zephaniah's message so full of judgment? The answer lies in the evil deeds of Josiah's grandfather, Manasseh (2nd Kings 23:26, and consider 2nd Kings 23:23-27 as a whole also) and in God's knowledge of the coming evils of Jehoahaz, Jehoiakim, Jehoiachin and Zedekiah. Still, it would seem that Zephaniah's message would have sounded awfully strange to his audience as he presented it during a time of peace and righteousness.
Messages about God's anger and judgment are seldom popular. If the people are guilty then they are offended because of their guilt. If the people are innocent, then they are offended for that reason. Yet, hellfire and brimstone messages must be preached anyway, for 3 reasons. First, they are true. God's judgment against sin is coming. Second, because God has commanded His servants to declare His wrath. Popular or not, it must be declared. Third, even in the best of times, there are seeds of sin that are hidden under the surface; sins that are ready to spring up and destroy all spiritual progress. The warnings of God's wrath can be useful in suppressing temptations and delaying (or preventing) spiritual decline.